Significant rainfall forecast, with high freezing levels. Veronica Woodruff explores what this means, in the context of the Lillooet River

Like all good Canadians I love the weather. Almost to point of obsession, especially when I think about it in the context of the hydrology of the Lillooet River. It is so interesting around here with the variability in the seasons and across different elevations. The first of the fall storms are forecast which makes me reflect on the dynamic system that affects us all.

The nature of a river like the Lillooet is to meander across a valley over time (Figure 1), which doesn’t create the best human settlement conditions.

Figure 1. Looking downstream at an undeveloped portion of the Lillooet River towards Pemberton Valley showing the river moving where it pleases.

In order to create better settlement conditions, significant flood protection investments were made in the late 1940’s including dyke construction, river straightening and dredging approximately 473,000 m3 out of Tenas and Lillooet Lake Narrows to lower lake levels. Here are a couple of slides to help visualize the works completed (used with permission of Kerr Wood Leidel and also available in a 2002 report[1]).

It is particularly cool to see an overlay of the current town center on the 1947 aerial photograph. This shows that most of town was built on a side channel of the Lillooet River.

Figure 3. 1947 aerial photo overlaid with current Village of Pemberton cadastral map.

Since the initial investment, the Pemberton Valley Dyking District (PVDD) continues to work to try and beat the formidable force that is Mother Nature and keep all of our feet dry. Strategies include dyke upgrades, flap gates, pump stations and dredging. As you can imagine, every time there is heavy rainfall, sediment (gravel and sand) is transported down the river. This sediment originates from the upper reaches of the Lillooet River valley where the riverbed gradient is higher, and as the slope of river flattens, the sediment begins to deposit along the river bottom. The highest deposition zone is between the one-lane forestry bridge in the Pemberton Meadows to the Green River Confluence downstream of the golf courses. The amount of sediment being deposited annually in this reach was initially assessed at 30,000 to 40,000 m2. Imagine 3000-4000 dump trucks unloading at the train bridge every summer during runoff season! This was reassessed in 2011 to assess the potential sediment increase resulting from the Mt. Meager landslide. This report ultimately concluded that the landslide does not significantly affect the amount of sediment transport[2] because there is basically an infinite supply of sediment from a number of sources and the Lillooet River is only capable of moving so much annually. The PVDD undertakes gravel removal at select locations to ensure the dykes retain capacity. It’s either that or continue to raise the dykes.

We can see this trend visually looking at the Lillooet River Gauge record of data where higher returns are more common now (Table 2). In fact, in 2015, we had the 3rd and 9th highest flows in a two-week period. The fact that we haven’t seen widespread flooding is a testament to the work the PVDD has been doing, but all of these big flows are hard on the infrastructure. And if you live on the south side of Highway 99 towards the Industrial Park, you know that more should be done after last year’s flood.

Table 1. Government of Canada Lillooet River Gauge 08MG0005 showing top 20 highest flows ever recorded. (Note 2016 data was collected from www.wateroffice.ec.gc.ca on November 11, 2016. Wateroffice isn’t showing instantaneous maximum flow for this date but only mean discharge (1000 m3/s) which is still subject to change)

In addition to floods, heavy rainfall also triggers debris flows (Figure 4), with some drainages more susceptible than others. If you are out hunting or mushroom picking this week it helps to understand your surroundings. With that debris, comes tons of wood deposited into the river which is always mesmerizing floating along the Lillooet River at the Highway 99 bridge (Figure 5).

Figure 4. South Creek debris flow, September 2015.

Figure 5. Strawberry Point after the 2003 flood. No shortage of freezers, coolers and North Arm pumpkins in the mix.

We once again have significant rainfall forecast, with high freezing levels.

Last year, the November storm (Figure 6) was a single event and occurred while the lake and river levels were very low, similar to today’s conditions. This is ideal because it gives the water a place to go. We get widespread issues when the lake is full and the river is at capacity and the rain is still coming, similar to the 2003 flood (Figure 7, Figure 8, Figure 9). Living on the Lillooet River bank is wonderful however, I feel more anxiety about early snowfall rather than my skier delight. Hopefully the dykes continue to hold and all my neighbor’s feet continue to stay dry.

Figure 6. Veronica’s backyard showing the height of the Lillooet River in November 2016.

Figure 7. Ryan River meets the Lillooet at the Riverlands Barn.

Figure 8. Rutherford Creek washout 2003 (I have no idea where I got these photos but I’ve had them forever, but if it is yours please let me know so I can ask permission, credit and/or remove).

Figure 9. Nairn Falls during 2003 flood. Have you ever stood on that platform? (same as above, let me know if these are your photos. I have been using this set to teach kids).